1. Field of the Invention
The present invention relates generally to semiconductor devices, and, more specifically, to a ring-resonator based optical isolator and circulator.
2. Description of the Related Art
As electronic devices increase in speed, and are more commonly used in optical systems, integration of electronic and optical semiconductor devices has become more common. Such circuits, often called photonic integrated circuits, have found uses in several consumer and commercial applications.
Proper integration of optical devices for inter-chip and intra-chip optical interconnections is important in the performance of the final integrated circuit. Some optical components, such as lasers, modulators, and photodetectors, can be monolithically integrated with electronic devices, however, other optical devices, such as optical isolators, are difficult to integrate with other electronic and optical devices.
Optical isolators are important devices in optical systems in that optical isolators minimize light reflections into transmitting lasers, and thus reduce instabilities and system noise in optical and electro-optical systems. Typically, optical isolators are integrated using a magneto-optical effect, and the mechanisms used to integrate optical isolators through magneto-optical effects are classified into three main categories: non-reciprocal Transverse Electric (TE)-Transverse Magnetic (TM) (TE-TM) mode conversion, non-reciprocal loss (NRL), and non-reciprocal phase shift (NRPS) devices.
Each type of device has associated performance and integration issues. A Nonreciprocal TE-TM mode converter in a waveguide creates an inherent phase mismatch between the TE and TM modes. In order to reach high conversion efficiency, phase matching is required between these two modes, however, zero birefringence is challenging to achieve in a waveguide.
A device employing the NRL technique has been demonstrated by combining a Semiconductor Optical Amplifier (SOA) and magneto-optical film, achieving a reasonable amount of isolation. However, the SOA requires continuous power consumption to compensate for the extra insertion loss due to the lossy magneto-optic material. Another approach to realize NRL is to guide backward light to a radiation mode, which increases optical loss and thus lowers performance.
NRPS devices has been embodied in a Mach-Zehnder Interferometer (MZI) for waveguide optical isolators, with a reasonable extinction ratio, however, these devices are typically sized on the order of millimeters, which makes monolithic integration difficult and expensive. NRPS combining microresonators for miniaturization of isolators has been proposed, which determines the size of the optical isolator by the size of the resonator, which can be on the order of microns, which remains difficult to integrate.
Problems associated with using discrete optical isolators incorporating such conversions are typically associated with performance of the system using such discrete devices. By integrating optical isolators with other photonic and electronic devices, performance can be improved and cost and size of the final circuitry is much smaller, thus creating additional applications for such integrated systems.
It can be seen, then, that there is a need in the art for optical isolators that are more readily integrated with other optical and electrical devices.